Preheat oven to 375°F and lightly grease a baking dish.
In a large bowl, whisk the eggs and milk until smooth.
Stir in tuna, cheese, onion, pepper, spinach, salt, pepper, and oregano.
Pour mixture into the prepared baking dish and spread evenly.
Bake for 30 to 35 minutes until the quiche is set and golden on top.
Let it cool for a few minutes before slicing and serving.